1. Field of the Invention
The present invention relates to a monochromator combining as light dispersing element one prism and at least one diffraction grating in series, and more particularly to a monochromator having its slit width so set as to cover a deviation of wavelength resulting from the variation of refractive index of the prism due to the variation of temperature and a deviation of wavelength resulting from the rotational error of the prism.
2. Description of the Prior Art
A conventional dispersion type monochromator using a diffraction grating and/or prism which serves as light dispersing element combines a plurality of light dispersing elements in order to achieve the higher resolving power or less stray light. A double monochromator using the two light dispersing elements is embodied, for example, by the combination of a diffraction grating and a prism; a diffraction grating and another diffraction grating; or a prism and another prism. The monochromator using the combination of the two diffraction gratings has the drawbacks that it needs a filter for screening light having the different wavelength of unnecessary orders and that it has a great loss of light amount because of the employment of the two diffraction gratings having a poor energy efficiency. The double monochromator using the combination of the two prisms, on the other hand, has the drawback that it cannot obtain a sufficient resolving power because of a poor dispersion coefficient although it has little stray light. In this respect, it is general that a double monochromator which combines the prism and diffraction grating is commonly used. The double monochromator of this kind is provided with an entrance slit, an intermediate slit and an exit slit other than the prism and diffraction grating.
In this double monochromator, it is important to arrange the prism and diffraction grating so that they may rotate in a precisely interlocking manner in order to obtain light of various needed wavelengths. The poor rotational accuracy in a single monochromator using a prism or a diffraction grating causes the degraded precision in wavelength of exit light but gives no influence upon the amount of exit light. In the double monochromator using the prism and diffraction grating, however, the poor rotational accuracy undesirably causes not only the poor precision of wavelength but the reduction in amount of exit light because the passing wavelength range of the intermediate slit deviates from that of the exit slit. For this reason, this double monochromator needs a precisely following rotational mechanism which allows the prism and diffraction grating to interlock precisely and rotate so that the same wavelength can pass through the slits. From the viewpoints of a technique and expense, however, it is difficult to improve the rotational accuracy more precisely than a certain limitation, so that the drawback of the rotational error actually remains and has not been solved satisfactorily.
It is necessary to scan the wavelength in a case where the above-mentioned double monochromator is used as spectrophotometer.
In the above-mentioned double monochromator the diffraction grating can be driven by means of a feed screw, for example, according to a sine-bar system to change the wavelength linearly. There is, on the other hand, no effective method but to make use of a drive by means of cams in order to change the wavelength linearly because the refractive index of the prism varies non-linearly with respect to the wavelength. It is, however, very troublesome to manufacture precise cams. Generally, the refractive index of the prism relative to the wavelength is experimentally determined, so that the refractive index relative to any wavelength can be found precisely only with difficulty. Finally, a method must be used in which the manufactured cam is mounted in the monochromator, then ground during the measurement of the wavelength of the exit light and finished so as to change the wavelength linearly. This method requires very troublesome operations. There is further another method in which a lever mounted on the rotational axis of the prism is elongated to form the cam to be large-sized and to increase the precision of the cam. However, this disadvantageously causes the monochromator itself to be large-sized.
A more serious problem is that the refractive index of the prism depends upon temperatures. The variation of the refraction index due to the variation of the temperature very undesirably causes the deviation of wavelength of the exit light as well as the reduction in amount of exit light.
The drawbacks of the double monochromator have been described above, but they appear not only in the double monochromator but also in a monochromator including the combination of more than three light dispersing elements.
In order to overcome the above-described drawbacks and to facilitate the manufacture of the monochromator using a plurality of light dispersing elements, a method can be proposed in which, for a monochromator having the prism at a position nearest to the entrance slit, the slit width of the entrance slit is made constant to a relatively large predetermined width and slit width of the intermediate slit and exit slit are made variable in an interlocking manner, while, for a monochromator using the prism at a position nearest to the exit slit, the slit width of the exit slit is made constant to a relatively large predetermined width and slit widths of the entrance slit and intermediate slit are made variable in an interlocking manner, thereby obtaining exit light having a needed wavelength bandwidth and amount of light. In the form method in which the width of the entrance slit is made constant to a certain value, the width of the entrance slit which is smaller than that of the two other slits causes not only the meaningless determination of the slit width because of a direct influence of the rotational error of the prism, but also the loss of light by an amount corresponding to the reduction of the slit width. The width of the entrance slit greater than that of the other two slits, or the other hand, allows the elimination of influence due to the rotational error of the prism and the prevention of the loss of light, but causes the reduction in resolving power. Further, a width of the entrance slit must greater than that of the other two slits undesirably causes much stray light to be produced. The stray light decreases in a wavelength range in which the prism has a good dispersion while it increases in an wavelength range in which it has a poor dispersion. It will, therefore, be understood that these methods also cannot eliminate the above-mentioned drawbacks satisfactorily.